Make sure we can still write data to txg

The final txgs are used only to clear out any remaining deferred
frees, and we cannot write new data to them. Make sure we do not
try to do so.

Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Mariusz Zaborski <mariusz.zaborski@klarasystems.com>
Closes openzfs#18139

Author: oshogbo

module/zfs/spa.c

@@ -2167,6 +2167,9 @@ spa_sync_time_logger(spa_t *spa, uint64_t txg)
 	if (curtime < spa->spa_last_flush_txg_time + spa_flush_txg_time) {
 		return;
 	}
+	if (txg > spa_final_dirty_txg(spa)) {
+		return;
+	}
 	spa->spa_last_flush_txg_time = curtime;
 
 	tx = dmu_tx_create_assigned(spa_get_dsl(spa), txg);